For example, in a machine for successively making plastic bags each of which includes a pattern printed thereon, a plastic film is fed longitudinally thereof. The plastic film includes the pattern printed thereon repeatedly, with a print pitch being predetermined. The plastic film is fed intermittently for a length corresponding to the print pitch and stopped temporarily whenever being fed intermittently. In general, plastic films are superposed with each other. The plastic films are then heat sealed with each other by a heat seal device and cross cut by a cross cutter whenever being fed intermittently and when being stopped temporarily, to successively make plastic bags.
In this case, conformity in pattern is required between the plastic films. On the other hand, minute errors must exist in the print pitches of plastic film to be accumulated, resulting in a discrepancy in pattern between the plastic films. In order to overcome this problem, there has been proposed an apparatus for correcting the print pitches, as disclosed in Japanese Patent No. 4,121,722.
In the apparatus of the patent, a plurality of optical sensors are disposed at a path along which the plastic films are fed, to detect and compare positions at which the patterns are printed on the plastic films repeatedly. In addition, the apparatus is arranged to heat a specific film partially to stretch it by tension at a position predetermined upstream of the optical sensors when the specific film goes ahead of other films. The apparatus can therefore correct the print pitches, conformity in pattern being obtained between the plastic films.
However, the apparatus can merely correct the print pitches of the plastic films relatively to each other. The apparatus cannot correct the print pitches individually to keep them at a fixed value. In addition, when stretching the plastic film, the print pitch is fluctuated by the stretching amount. The plastic films are then heat sealed with each other and cross cut by the heat seal device and the cross cutter at a position predetermined with respect to the pattern. The plastic bag must therefore have a size changed by the print pitch fluctuated. The apparatus cannot keep the size constant. Furthermore, the heat seal device and the cross cutter have to be moved by a drive means for adjustment of position when operating the machine so that the plastic films should be heat sealed and cross cut at the position predetermined with respect to the pattern, resulting in technical problems.
It has also been proposed in the patent to keep an average of print pitches constant by correcting the print pitches. However, the apparatus cannot correct the print pitches individually all the same. It can merely keep the average of print pitches.
By the way, the apparatus is arranged to heat the plastic film partially to stretch it by tension, as described previously. However, the plastic film is not always suitable for heating. There has therefore been proposed a device for stretching a plastic film in a different way, as disclosed in Japanese Patent Publication No. 4,461,201. The device can stretch a plastic film by using rollers without heating the plastic film partially.
It is therefore an object of the invention to provide an apparatus for correcting print pitches at which a pattern is printed on a plastic film repeatedly, which can correct the print pitches individually.